Adverse (pressure gradient) fluid flows generated over aerodynamic surfaces can buffet and fatigue any downstream structures so exposed. Additionally, such flows can affect efficiency by increasing drag or resistance over the surface. Such adverse fluid flows can be generated at the fore body of an aircraft or other upstream structure, and damage control surfaces, engines, after body/empennage, nacelles, turrets, or other structures integrated into the airframe. Additionally, these adverse fluid flows can be ingested within engine air intakes or other like air inlets leading to poor performance and/or stalling of the aircraft engines. Stalling the aircraft engine creates a potentially hazardous condition.
Next generation aircraft, such as blended wing body, compound this problem by incorporating gas turbine inlets with serpentine spines within the air frame. Additionally, exotic aperture shapes for the inlet and outlet may cause excessive propulsion performance losses. These losses emanate from strong secondary flow gradients in the near wall boundary of the airflow, which produce coherent large-scale adverse fluid flows.
In the past, aircraft components were designed to minimize the strength of adverse pressure gradient flow fields to reduce the extent of or eliminate the separation of boundary layer flow from aircraft surfaces to reduce the destructive structural impact of separated flow on aircraft components and performance. This approach limits design options and increases vehicle size, weight and cost. Alternatively, the components in the path of the adverse fluid flows were structurally hardened or replaced more frequently to avoid failures resulting from these stresses. Placing components, such as engines or control surfaces, in non-optimal positions in order to reduce these stresses often results in reduced vehicle performance. Similarly, adding structural weight to support increased stress loads caused by the flow field vortices also results in reduced vehicle performance.
Another solution employs active or passive control flows to mitigate the effects of the adverse flow fields. However, these control flows create a need for compressed air and piping to bring the control jets to regions requiring flow-control authority. These control jets then manipulate the boundary layer with induced mixing between the primary fluid flow and the secondary fluid flow. This solution also adds structural weight to supply and support the control jets that result in reduced vehicle performance.
In either of the above described solutions, mixing is promoted by vortices trailing longitudinally near the edge of the boundary layer. Fluid particles with high momentum in the stream direction are swept along helical paths toward the aircraft surfaces to mix with and, to some extent replace low momentum boundary layer flow. This is a continuous process that provides a source to counter the natural deceleration of the flow near a solid surface in a boundary layer that can lead to flow separation in regions with adverse pressure gradients and low energy secondary flow accumulation.
To avoid the increased weight of the supply system for control jets, synthetic jets may be employed. These synthetic jets may be those described in U.S. Pat. No. 6,722,581 entitled “SYNTHETIC JET ACTUATORS,” which is hereby incorporated by reference. Synthetic jets, which may be large scale devices or small scale Micro-fabricated Electro-Mechanical Systems (MEMS) devices, are known to influence the flow over a surface, for example to control flow separation on an airfoil. A typical synthetic jet actuator comprises a housing defining an internal chamber. An orifice is present in a wall of the housing. The actuator further includes a mechanism in or about the housing for periodically changing the volume within the internal chamber so that a series of fluid vortices are generated and projected into an external environment beyond the orifice of the housing. Various volume changing mechanisms are known, for example a piston positioned in the jet housing to move so that fluid is moved in and out of the orifice during reciprocation of the piston, or a flexible diaphragm as a wall of the housing. The fluid moved may be either a liquid or gas. The flexible diaphragm is typically actuated by a piezoelectric actuator or other appropriate means.
Typically, a control system is utilized to create time-harmonic motion of the diaphragm. As the diaphragm moves into the chamber, decreasing the chamber volume, fluid is ejected from the chamber through the orifice. As the fluid passes through the orifice, the flow separates at the sharp edges of the orifice and creates vortex sheets which roll up into vortices. These vortices move away from the edges of the orifice under their own self-induced velocity. As the diaphragm moves outward with respect to the chamber, increasing the chamber volume, ambient fluid is drawn from large distances from the orifice into the chamber. Since the vortices are already removed from the edges of the orifice, they are not affected by the ambient fluid being drawn into the chamber. As the vortices travel away from the orifice, they synthesize a jet of fluid, a “synthetic jet,” through entrainment of the ambient fluid.
However, these devices have relatively limited capacity, in that moving elements are limited in power and/or deflection unless driven by a large, heavy electromechanical device which is impractical for most aircraft applications. Although high-amplitude high-frequency jets may be created synthetically, application of these devices has been restricted due to the inability to generate sufficient pressure to choke the flow at the jet orifice. This is the condition necessary to create sonic flow at the orifice. Therefore, it would be desirable to obtain increased performance of synthetic jet actuators in such environments. Accordingly, there is a need for a synthetic jet actuator having greater capacity than previous devices.